Auto merge of rust-lang#67925 - petertodd:2020-fromstr-infallible, r=LukasKalbertodt

Change FromStr for String to use Infallible directly

Fixes the confusing documentation on `ParseError` by making it irrelevant.

It might be fine to mark it as depreciated right now too - I can't imagine much code uses `ParseError` directly.

Author: bors

src/liballoc/string.rs

@@ -2106,26 +2106,19 @@ impl ops::DerefMut for String {
     }
 }
 
-/// An error when parsing a `String`.
+/// A type alias for [`Infallible`].
 ///
-/// This `enum` is slightly awkward: it will never actually exist. This error is
-/// part of the type signature of the implementation of [`FromStr`] on
-/// [`String`]. The return type of [`from_str`], requires that an error be
-/// defined, but, given that a [`String`] can always be made into a new
-/// [`String`] without error, this type will never actually be returned. As
-/// such, it is only here to satisfy said signature, and is useless otherwise.
+/// This alias exists for backwards compatibility, and may be eventually deprecated.
 ///
-/// [`FromStr`]: ../../std/str/trait.FromStr.html
-/// [`String`]: struct.String.html
-/// [`from_str`]: ../../std/str/trait.FromStr.html#tymethod.from_str
+/// [`Infallible`]: ../../core/convert/enum.Infallible.html
 #[stable(feature = "str_parse_error", since = "1.5.0")]
 pub type ParseError = core::convert::Infallible;
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl FromStr for String {
     type Err = core::convert::Infallible;
     #[inline]
-    fn from_str(s: &str) -> Result<String, ParseError> {
+    fn from_str(s: &str) -> Result<String, Self::Err> {
         Ok(String::from(s))
     }
 }